1. Field of the Invention
This invention relates to an auto strobe control circuit in which the luminance of an object exposed to light emitted from a flash tube is log-converted and subjected to an arithmetic operation.
2. Description of the Prior Art
Auto strobes used for photographic purposes measure the luminance of an object exposed to light emitted from a flash tube and stops flashing of the tube when the amount of light reflected from the object reaches a predetermined value, thereby controlling the flashing duration to obtain a predetermined amount of light on the image surface.
In many conventional auto strobes, the light reflected from an object is converted to a photoelectric current, which is then directly integrated by a capacitor. Therefore, the conventional auto strobes are disadvantageous in that the measuring range is limited to the order of 10.sup.2 and there is a limitation on the usable ranges of aperture and film sensitivity.
Under these circumstances, it has been proposed in Japanese Unexamined Patent Publication No. 54(1979)-73029 to log-convert and arithmetically process the luminance of the object, thereby to widen the measuring range from the order of 10.sup.2 to 10.sup.6.
The auto strobe control circuit disclosed in Japanese Unexamined Patent Publication No. 54(1979)-73029 is very convenient for practical purposes because wider ranges of apertures and film sensitivities can be used by widening the measuring range by means of log-conversion. In this auto strobe control circuit, a log-compression circuit comprising a log diode connected to a feedback circuit coupling the negative input terminal of an operational amplifier with the output terminal thereof is used as a means for log-conversion. However, the log-compression circuit is not suitable for use in a strobe photometric circuit because the response time is too long (for example, several tens of microseconds) due to the operation time in the operational amplifier. Accordingly, the auto strobe control device disclosed in Japanese Unexamined Patent Publication No. 54(1979)-73029 involves a risk of malfunctioning due to the low response characteristics attributable to the opetation circuit in the log-compression circuit.